To use the sink in the kitchen, you need to make sure you have water left in the freshwater tank. You can see how much water is left by looking at the water fill line inside the vehicle next to the sink.
To use the sink, make sure to turn the water pump. You can switch it on on the panel above the back seat:
Or directly on the tank on the back of the car:
You can use the sink the same way you would at home.
Make sure to switch it back off in the same place after you are done using it!